Experiments at the Intersection of Inertially Confined Fusion and Astrophysics: An Investigation of an Asymmetrically Irradiated Foam Sphere

Abstract

The motivation for this thesis was to develop a laboratory astrophysics platform to investi- gate how a gas cloud of predefined optical depth would evolve when exposed to radiation from a nearby star. A scaled experiment was devised to mock up this multi-parsec astro- physical system on Earth. A laser-irradiated, thin-gold foil mocked up radiation from a star and a foam sphere mocked up a nearby gas cloud. The systems’ optical depth linked the general evolution of the scaled experiment to that of the astrophysical phenomena. The optical depth of the foam sphere in the experiment was controlled by the density and composition of the foam. The initial experiments, reported here, describe the results for optically-thick spheres. Radiographic evidence of the formation of an interface between rarefying gold and carbon plasmas suggest that the experimental measurements have implications for inertially confined fusion (ICF) hohlraums in addition to the astrophysics. In an ICF hohlraum, lasers heat a high-Z wall, which creates an x-ray source and rarefies high-Z plasma inwards. Meanwhile, the x-rays generated by laser-heated plasma ablate the low-Z, outer layer of the fusion capsule outwards. In this manner, a counter-propagating flow of high-Z and low-Z plasma is created similar to that of the experiment discussed in this thesis.

In this thesis, the results of a radiation transport experiment conducted on OMEGA-60 where x-rays, from a laser-irradiated, thin-gold foil, asymmetrically illuminated a precisely positioned nearby, carbon-foam sphere are presented. A snapshot of the gold foil and sphere motions in each experimental shot were recorded by two, orthogonal-axes of x-ray radiography on independently timed CCD detectors. The nominal edge-on-view of the gold foil allows for radiographic measurements of the interaction between the rarefied gold and carbon plasmas similar to those found in a hohlraum. This system is also reminis- cent of a large star irradiating nearby gas clouds. The time series of resultant radiographic images showed the formation of an interface between the rarefied gold and carbon plasmas, a shock moving into the sphere, and a blunting of the initial sphere’s shape. Measurements made along a horizontal lineout through the initial sphere’s center showed that the shock moved linearly around 64 um/ns into the sphere, the gold-carbon interface formed by 2 ns at the sphere edge and did not move much. The deformation of the sphere was driven by the incident radiation and not by mechanical pressures applied by gold plasma. The blunting of the sphere was likely due to the geometric reduction of flux near the sphere’s poles, which reduced compression near the sphere’s poles. Higher x-ray flux near the sphere’s equator caused high compression and a faster shock, which flattened the sphere. This experiment suggests that flux from a star alone is not enough to collapse a gas cloud and form more stars.